Plasma processing apparatus

ABSTRACT

A microwave plasma processing apparatus includes a processing chamber; a microwave source that outputs a microwave; a dielectric plate that radiates the microwave output from the microwave source to the inside of the processing chamber; and a metal electrode provided on the side of a plasma-facing surface of the dielectric plate so as to be adjacent to the dielectric plate. Here, a part of the dielectric plate is exposed to the inside of the processing chamber at an outside of the metal electrode. Further, a cell area is defined as a virtual area that divides the ceiling surface of the processing chamber and is formed by two straight lines parallel to one diagonal line of the metal electrode and two straight lines parallel to the other diagonal line of the metal electrode and the cell area is a minimum rectangular area including the metal electrode and the dielectric plate.

TECHNICAL FIELD

The present invention relates to a plasma processing apparatus for performing a plasma process on a processing target object by exciting a gas by an electromagnetic wave.

BACKGROUND ART

Among various kinds of plasma generated by using an electromagnetic wave, microwave plasma is generated by introducing a microwave into a depressurized processing chamber through a dielectric plate (see, for example, Patent Document 1). In a microwave plasma processing apparatus, if an electron density n_(e) of plasma is higher than a cut-off density n, the microwave may propagate between a dielectric plate and the plasma without flowing into the plasma. While the microwave is propagating, a part of the microwave is absorbed into the plasma as an evanescent wave and is used to maintain the plasma. Such a microwave propagating between the dielectric plate and the plasma is referred to as, e.g., a dielectric surface wave.

-   Patent Document 1: Japanese Patent Laid-open Publication No.     2006-310794

If an electromagnetic wave of a low frequency is supplied to the plasma processing apparatus, a surface wave (hereinafter, referred to as a “metal surface wave” (“conductor surface wave”)) propagating between an inner metal surface of a processing chamber and the plasma may be generated in addition to the dielectric surface wave propagating between the dielectric plate and the plasma. The metal surface wave cannot propagate if the electron density of the plasma is lower than twice the cut-off density n_(c). The cut-off density n_(c) is proportional to the square of the frequency of the electromagnetic wave. Thus, unless the frequency of the metal surface wave is low and the electron density of the plasma is high, the metal surface wave may not propagate. Further, as the frequency of the metal surface wave is reduced, it becomes more difficult to attenuate the metal surface wave.

At a frequency of about 2450 MHz generally used for plasma generation, the cut-off density n_(c) reaches about 7.5×10¹⁰ cm⁻³. Accordingly, the metal surface wave may not propagate unless the electron density is equal to or higher than about 1.5×10¹¹ cm⁻³. By way of example, in case of low-density plasma having an electron density of about 1×10¹¹ cm⁻³ in a surface area thereof, the metal surface wave may not propagate at all. Meanwhile, in case that the electron density becomes higher, propagation of the metal surface wave is not remarkable because the metal surface wave is greatly attenuated. In the meantime, at a frequency of, e.g., about 915 MHz, the metal surface wave may propagate on the inner surface of the processing chamber for a long time even if the electron density in the surface area is about 1×10¹¹ cm⁻³, i.e., the plasma density is low.

Accordingly, in order to perform the plasma process by using an electromagnetic wave of a low frequency, an apparatus design for controlling the propagation of the metal surface wave as well as the propagation of the dielectric surface wave is required. For example, if a distribution of a standing wave of a metal surface wave on a surface of a metal electrode is different from a distribution of a standing wave of a metal surface wave on a surface of a metal cover, which is located in the vicinity of the metal electrode and has a similar shape to the metal electrode, and if there is a great difference in the distributions of the standing waves of both cases, an electric field energy on a ceiling surface of the processing chamber may be non-uniformly distributed. Moreover, if a height difference portion or a groove is present between the metal electrode and the meal cover, it may be difficult for a gas to flow in the vicinity of the height difference portion or the groove and the gas may stay there, resulting in generation of unstable plasma. Furthermore, when the dielectric plate or the metal electrode is fixed to the ceiling surface of the processing chamber by using a screw, a gap may be formed in a microwave transmission path, resulting in an increase of reflection of the microwave from the plasma and deterioration of energy supply efficiency of the microwave. Since these problems may affect uniformity and stability of the plasma, it is required to generate plasma uniformly and stably by optimizing the layouts, sizes, positions and the design of the metal electrode and its adjacent members.

DISCLOSURE OF THE INVENTION Problems to Be Solved by the Invention

In view of the foregoing, the present invention provides a plasma processing apparatus including a metal electrode and its adjacent members having optimal configurations for controlling propagation of a surface wave.

Means for Solving the Problems

In accordance with one aspect of the present invention, there is provided a plasma processing apparatus including a processing chamber configured to excite therein a gas and perform a plasma process on a processing target object; an electromagnetic wave source installed outside the processing chamber and configured to output an electromagnetic wave; a dielectric plate provided adjacent to a ceiling surface within the processing chamber and configured to radiate the electromagnetic wave output from the electromagnetic wave source to the inside of the processing chamber; and a rhombus-shaped metal electrode provided on the side of a plasma-facing surface of the dielectric plate so as to be adjacent to the dielectric plate. Here, a part of the dielectric plate is exposed to the inside of the processing chamber at an outside of the metal electrode. Further, a cell area is defined as a virtual area that divides the ceiling surface of the processing chamber and is formed by two straight lines parallel to one diagonal line of the metal electrode and two straight lines parallel to the other diagonal line of the metal electrode and the cell area is a minimum rectangular area including the metal electrode and the dielectric plate. Shapes of the metal electrode and the dielectric plate are designed such that a ratio of a length of a long side of the cell area to a length of a short side thereof is equal or less than about 1.2.

With this configuration, the electromagnetic wave output from the electromagnetic wave source is radiated into the processing chamber at the outside of the rhombus-shaped metal electrode after passing through the dielectric plate provided adjacent to the ceiling surface within the processing chamber. The cell area is defined as a virtual area that divides the ceiling surface of the processing chamber and is formed by two straight lines parallel to one diagonal line of the metal electrode and two straight lines parallel to the other diagonal line of the metal electrode and the cell area is a minimum rectangular area including the metal electrode and the dielectric plate, and shapes of the metal electrode and the dielectric plate are designed such that a ratio of a length of a long side of the cell area to a length of a short side thereof is equal or less than about 1.2.

In order to generate uniform plasma, distributions of standing waves of metal surface waves on the surfaces of the metal electrode and the metal cover are required to be uniform and there is a need of no great difference in the distributions of the standing waves in respective cases. If the cell area has a regular quadrilateral shape, same patterns of standing waves are formed on the surfaces of the metal electrode and the metal cover. It is because metal surface waves having same phases and same magnitudes are respectively supplied from corresponding positions around the metal electrode and the metal cover.

Meanwhile, if the cell area has a rectangular shape, different patterns of standing waves are formed on the surfaces of the metal electrode and the metal cover. It is because phases and magnitudes of the metal surface waves supplied from corresponding positions around the metal electrode and the metal cover are not same. If the symmetry of the standing waves of the metal surface waves on the surfaces of the metal electrode and the metal cover is poor, uniform control of the electric field intensity may become difficult, making it difficult to generate uniform plasma. In order to generate uniform plasma, an average value of an electric field intensity ratio of the metal surface waves at corresponding positions on the surfaces of the metal electrode and the metal cover is required to be equal to or smaller than about 1.5, more desirably, equal to or smaller than about 1.1.

FIG. 4 shows a relationship between an aspect ratio of a cell area and an electric field intensity ratio of metal surface waves at corresponding positions. FIG. 4 provides a result calculated by a simulation of electromagnetic field. As can be seen from the result of FIG. 4, in order to set the electric field intensity to about 1.5 or less, the aspect ratio of the cell area needs to be equal to or less than about 1.2. In this way, the distributions of the standing waves of the metal surface waves on the surfaces of the metal electrode and the metal cover can be uniformized or approximated. As a result, there is generated no great difference in the distributions of the standing waves in each case, so that uniform plasma can be generated.

In order to set the electric field intensity ratio to be equal to or less than about 1.1, the aspect ratio of the cell area needs to be equal to or less than about 1.1. In this way, the distributions of the standing waves of the metal surface waves on the surfaces of the metal electrode and the metal cover can be more uniformized or approximated.

In accordance with another aspect of the present invention, there is provided a plasma processing apparatus including a processing chamber configured to excite therein a gas and perform a plasma process on a processing target object; an electromagnetic wave source installed outside the processing chamber and configured to output an electromagnetic wave; a dielectric plate provided adjacent to a ceiling surface within the processing chamber and configured to radiate the electromagnetic wave output from the electromagnetic wave source to the inside of the processing chamber; a metal electrode provided on the side of a plasma-facing surface of the dielectric plate so as to be adjacent to the dielectric plate; and a metal cover having the same shape as or a similar shape to the metal electrode and provided on an area of the ceiling surface of the processing chamber where the dielectric plate is not provided. Here, a part of the dielectric plate is exposed to the inside of the processing chamber at an outside of the metal electrode. Further, a filling dielectric member is provided in a groove between the metal electrode and the metal cover.

With this configuration, the electromagnetic wave output from the electromagnetic wave source is radiated into the processing chamber at the outside of the metal electrode after passing through the dielectric plate provided adjacent to the ceiling surface within the processing chamber. A metal cover having the same shape as or a similar shape to the metal electrode is provided on the area of the ceiling surface of the processing chamber where the dielectric plate is not provided. Further, the filling dielectric member is provided in the groove between the metal electrode and the metal cover.

In the groove (Gap) between the metal electrode and the metal cover, a gas is difficult to flow but readily stay therein. By way of example, in a plasma cleaning process, it may be difficult for a cleaning gas to enter the gap, and, thus, a film adhering to an inner surface of the gap may not be removed. Further, since the groove-shaped gap is surrounded by walls in three directions, a plasma electron density may easily decrease, and, thus, it may become difficult to maintain stable plasma having constant density. Since metal surfaces waves are supplied from the dielectric plates between the metal electrode and the metal cover, the entire plasma may become unstable and non-uniform unless the stable plasma having constant density is maintained in the gap. In accordance with the present invention, however, the dielectric member is filled in the groove between the metal electrode and the metal cover in order to prevent these problems.

In accordance with still another aspect of the present invention, there is provided a plasma processing apparatus including a processing chamber configured to excite therein a gas and perform a plasma process on a processing target object; an electromagnetic wave source installed outside the processing chamber and configured to output an electromagnetic wave; a dielectric plate provided adjacent to a ceiling surface within the processing chamber and configured to radiate the electromagnetic wave output from the electromagnetic wave source to the inside of the processing chamber; a metal electrode provided on the side of a plasma-facing surface of the dielectric plate so as to be adjacent to the dielectric plate; a protrusion having the same shape as or a similar shape to the metal electrode and provided on an area of the ceiling surface of the processing chamber where the dielectric plate is not provided. Here, a part of the dielectric plate is exposed to the inside of the processing chamber at an outside of the metal electrode. Further, a filling dielectric member is provided in a groove between the metal electrode and the protrusion.

With this configuration, since the dielectric member is filled between the metal electrode and the protrusion, there is no space where a gas is difficult to flow but easily stay. Thus, uniform plasma can be generated stably.

A side cover may be provided around the metal electrode, and the filling dielectric member may be provided in a groove between the metal electrode and the side cover.

The filling dielectric member may be buried in the groove, or may be used to planarize the groove, or may be protruded from the groove.

The filling dielectric member may be provided so as to surround the outside of the metal electrode and have protruded portions from the groove.

The filling dielectric member may be protruded from the groove in the vicinity of a center of each side of the metal electrode.

The protruded portion of the filling dielectric member in the vicinity of a center of each side of the metal electrode may be higher than the protruded portion in the vicinity of a vertex of the metal electrode.

The protruded portions of the filling dielectric member may be arranged to have point symmetry with respect to a center of the metal electrode.

The filling dielectric member may be made of the same material as that of the dielectric plate.

In accordance with still another aspect of the present invention, there is provided a plasma processing apparatus including a processing chamber configured to excite therein a gas and perform a plasma process on a processing target object; an electromagnetic wave source installed outside the processing chamber and configured to output an electromagnetic wave; a dielectric plate provided adjacent to a ceiling surface within the processing chamber and configured to radiate the electromagnetic wave output from the electromagnetic wave source to the inside of the processing chamber; and a metal electrode provided on the side of a plasma-facing surface of the dielectric plate so as to be adjacent to the dielectric plate. Here, a part of the dielectric plate is exposed to the inside of the processing chamber at an outside of the metal electrode. Further, the metal electrode is fixed to the ceiling surface within the processing chamber by a plurality of first screws and a plurality of second screws different from the first screws, the plurality of first screws are configured to fix the metal electrode at positions having point symmetry with respect to a center of the metal electrode, and the plurality of second screws are arranged at positions having point symmetry with respect to the center of the metal electrode and are configured to fix the metal electrode at positions different from the positions of the plurality of first screws.

With this configuration, the electromagnetic wave output from the electromagnetic wave source is radiated into the processing chamber at the outside of the metal electrode after passing through the dielectric plate provided adjacent to the ceiling surface within the processing chamber. The metal electrode is fixed to the ceiling surface within the processing chamber by the plurality of first screws and the plurality of second screws different from the first screws.

The metal electrode is fixed to the processing chamber via the dielectric plate by the plurality of first screws and the plurality of second screws. To be specific, the plurality of first screws are arranged at same intervals and equi-spaced from the center of the metal electrode, and the plurality of second screws are equi-spaced from the center of the metal electrode and are arranged at positions different from the positions of the four first screws.

In this way, the metal electrode is fixed by the two kinds of screws arranged to have point symmetry with respect to the center of the metal electrode without having electrical or mechanical deviation. Accordingly, the dielectric plate serving as a microwave transmission path has no gap. Thus, a wavelength and a propagation speed of the microwave may not be varied and there is no difference between actual impedance and a design value. As a result, reflection of the microwave from plasma would be reduced and energy supply efficiency of the microwave can be improved. Especially, depending on the diameters or positions of the screws, a difference in the distribution of electric field intensity of the metal electrode can be reduced, so that uniform plasma can be generated stably. Further, heat from the plasma can be transferred to the cover body 300 through the screws.

Diameters of the plurality of first screws may be smaller than diameters of the plurality of second screws.

The number of the plurality of first screws may be four and the four first screws may be located on diagonal lines of the metal electrode.

The number of the plurality of second screws may be four and the four second screws may be located at positions closer to the center of the metal electrode than the four first screws.

If the metal electrode is of a regular quadrilateral shape, each of the four second screws may be installed at a position equi-spaced from two adjacent first screws among the four equi-spaced first screws.

The dielectric plate may be exposed in a substantially strip shape to the ceiling surface within the processing chamber at the outside of the metal electrode.

The number of the dielectric plate and the number of the metal electrode are plural, the dielectric plates may be sandwiched between the metal electrodes and the ceiling surface of the processing chamber, and the metal electrodes are regularly arranged on the ceiling surface such that vertexes of adjacent metal electrodes are positioned closest to each other.

Effect of the Invention

As discussed above, in accordance with the embodiments of the present invention, configurations of the metal electrode and its adjacent members can be optimized, so that the propagation of the surface wave can be controlled efficiently.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a longitudinal cross sectional view of a microwave plasma processing apparatus (cross sectional view taken along a line 2-O-O′-2 of FIG. 2) in accordance with an embodiment of the present invention.

FIG. 2 is a cross sectional view taken along a line 1-1 of FIG. 1.

FIG. 3A provides a simulation result showing standing wave patterns of metal surface waves on surfaces of a metal electrode and a metal cover.

FIG. 3B provides a simulation result showing standing wave patterns of metal surface waves on the surfaces of the metal electrode and the metal cover.

FIG. 4 is a diagram showing a relationship between an aspect ratio of a cell area and an electric field intensity ratio of a metal surface wave at a corresponding position.

FIG. 5 is a diagram for describing a distribution of electric field intensity when a filling dielectric member is not provided between a metal electrode and a metal cover.

FIG. 6A is a diagram for describing a distribution of electric field intensity when a filling dielectric member is provided between a metal electrode and a metal cover.

FIG. 6B is a diagram for describing a distribution of electric field intensity when a filling dielectric member is provided between the metal electrode and the metal cover.

FIG. 6C is a diagram for describing a distribution of electric field intensity when a filling dielectric member is provided between the metal electrode and the metal cover.

FIG. 7 illustrates a metal electrode and its adjacent members provided at a ceiling surface.

FIG. 8 is a bottom view of FIG. 7.

FIG. 9 is a cross sectional view taken along a line 3-3 of FIG. 8.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. Through the present specification and drawings, parts having substantially same function and configuration will be assigned same reference numerals, and redundant description will be omitted.

(Configuration of Plasma Processing Apparatus)

A configuration of a microwave plasma processing apparatus (MSEP: Metal Surface wave Excitation Plasma) in accordance with an embodiment of the present invention will be explained with reference to FIG. 1. FIG. 1 is a schematic longitudinal cross sectional view of a microwave plasma processing apparatus 10. FIG. 1 shows a cross sectional view taken along a line 2-O-O′-2 of FIG. 2. FIG. 2 illustrates a ceiling surface of the microwave plasma processing apparatus 10. FIG. 2 is a cross sectional view taken along a line 1-1 of FIG. 1.

(Outline of Microwave Plasma Processing Apparatus)

As depicted in FIG. 1, the microwave plasma processing apparatus 10 may include a processing chamber 100 for performing therein a plasma process on a glass substrate (hereinafter, simply referred to as a “substrate G”). The processing chamber 100 may include a chamber main body 200 and a cover body 300. The chamber main body 200 has a cube shape having a bottom and a top opening, and the opening is closed by the cover body 300. The cover body 300 may include an upper cover body 300 a and a lower cover body 300 b. An O-ring 205 is provided at a contact surface between the chamber main body 200 and the lower cover body 300 b, so that the chamber main body 200 and the cover body 300 are hermetically sealed and a processing space is formed. An O-ring 210 and an O-ring 215 are provided at a contact surface between the upper cover body 300 a and a lower cover body 300 b, so that the upper cover body 300 a and a lower cover body 300 b are hermetically sealed. The chamber main body 200 and the cover body 300 are made of a metal such as aluminum alloys and are electrically grounded.

A susceptor (stage) 105 for mounting thereon the substrate G is provided within the processing chamber 100. The susceptor 105 is made of, e.g., aluminum nitride. The susceptor 105 is held on a support 110, and a baffle plate 115 for controlling a flow of a gas within the processing space in a desirable state is provided around the susceptor 105. Further, a gas exhaust pipe 120 is provided in the bottom of the processing chamber 100, and the gas within the processing chamber 100 is exhausted by a vacuum pump (not shown) provided outside the processing chamber 100.

Referring to FIGS. 1 and 2, metal electrodes 310 and metal covers 320 are regularly arranged on the ceiling surface of the processing chamber 100. Eight metal electrodes 310 are regularly arranged on the ceiling surface within the processing chamber such that vertexes of adjacent metal electrodes 310 are positioned closest to each other. Three metal covers 320 are provided at places where the metal electrodes 310 are not provided. The metal covers 320 are arranged on the ceiling surface within the processing chamber 100 such that their vertexes are positioned closest to the vertexes of their adjacent metal electrodes 310. Further, a side cover 350 for surrounding the metal electrodes 310 and the metal covers 320 is provided at the outside of the eight metal electrodes 310.

Each metal electrode 310 is provided on the side of a plasma-facing surface (bottom surface) of a dielectric plate 305 so as to be adjacent to the dielectric plate 305. Each metal electrode 310 may be a rhombus-shaped flat plate and a part of the dielectric plate 305 is exposed to the inside of the processing chamber 100 at the outside of the metal electrode 310. Here, a rhombus shape refers to a quadrilateral having four sides of same lengths and includes a regular quadrilateral.

Each dielectric plate 305 is larger than the electrode metal 310 and the dielectric plate 305 may be an octagonal flat plate with chamfered corners at the vertexes of the metal electrode 310. The dielectric plate 305 is provided adjacent to the ceiling surface within the processing chamber while being sandwiched between the ceiling surface and the metal electrode 310. The dielectric plate 305 is exposed in a substantially strip shape at the outside of the metal electrode 310 on the ceiling surface within the processing chamber. The dielectric plate 305 is capable of radiating a microwave output from a microwave source 900 into the processing chamber through the strip-shaped portion.

Eight dielectric plates 305 and eight metal electrodes 310 are arranged at a same pitch at positions inclined by about 45° with respect to a substrate G or the processing chamber 100. The pitch is set such that a diagonal length of one dielectric plate 305 is at least about 0.9 times larger than a distance between centers of adjacent dielectric plates 305. Accordingly, chamfered corners of the dielectric plates 305 are positioned adjacent to each other.

Each metal cover 320 is provided on the ceiling surface of the processing chamber 100 at a place where the dielectric plate 305 is not provided. The metal cover 320 may have the same shape as or a similar shape to the metal electrode 310. The metal cover 320 is thicker than the dielectric plate 305 by a thickness of the dielectric plate 305. A filling dielectric member 315 may be filled in a groove between the metal electrode 310 and the metal cover 320. With this configuration, the ceiling surface becomes flat, and recesses at positions where the dielectric plates 305 are exposed are filled up, so that substantially the same pattern of the metal electrodes 310 and their adjacent members may be repeatedly arranged.

The dielectric plate 305 may be made of alumina, and the metal electrode 310, the metal cover 320 and the side cover 350 may be made of aluminum alloys. In the present embodiment, although the eight dielectric plates 305 and the eight metal electrodes 310 are arranged in a matrix pattern of two columns and four rows, the number of the dielectric plates 305 and the metal electrodes 310 may be increased or reduced without being limited to the example configuration.

Referring to FIG. 2, the dielectric plates 305 and the metal electrodes 310 are fixed to the cover body 300 by first screws 380 and second screws 390. The metal covers 320 and the side cover 350 are also fixed to the cover body 300 by first screws 380 and second screws 390. A main gas flow passage 330 is formed between the upper cover body 300 a and the lower cover body 300 b. A gas within the main gas flow passage 330 flows into first gas flow passages 325 a formed within a multiple number of first screws 380, and a slim tube 335 for narrowing a passageway is fitted in an inlet of each first gas flow passage 325 a. The slim tube 335 is made of, e.g., ceramics or a metal. Further, a second gas flow passage 310 a 1 is formed between the metal electrodes 310 and the dielectric plates 305, and second gas flow passages 320 a 1 and 320 a 2 are formed between the metal covers 320 and the cover body 300 and between the side cover 350 and the cover body 300, respectively. Leading end surfaces of the first and second screws 380 and 390 are positioned on the same surface as bottom surfaces of the metal electrodes 310, the metal covers 320 and the side cover 350 in order not to disturb a plasma distribution. First gas discharge holes 345 a formed in the metal electrodes 310 and second gas discharge holes 345 b 1 and 345 b 2 respectively formed in the metal covers 320 and the side cover 350 are opened downward at a uniform pitch. Meanwhile, the first and second screws 380 and 390 may be formed as one body with the metal electrodes 310, the metal covers 320 and the side cover 350.

A gas supplied from a gas supply source 905 is flown into the first gas flow passages (branch gas flow passages) 325 a via the main gas flow passage 330 and then is supplied into the processing space through the first gas discharge holes 345 a and the second gas discharge holes 345 b 1 and 345 b 2 via the second gas flow passages 310 a 1 in the metal electrodes 310 and via the second gas flow passages 320 a 1 and 320 a 2 in the metal covers 320 and the side cover 350. An O-ring 220 is installed on a contact surface between the lower cover body 300 b and the dielectric plate 305 in the vicinity of a periphery of a first coaxial waveguide 610, so that the air within the first coaxial waveguide 610 may not enter the processing chamber 100.

In this way, by forming a gas shower plate in the metal surface at the ceiling of the processing chamber, it is possible to suppress etching of surfaces of the dielectric plates by ions in the plasma and deposition of reaction products on an inner wall of the processing chamber that have occurred conventionally. Accordingly, contamination or particle generation can be reduced. Further, since the machining process for the metal can be easily performed unlike the dielectric material, manufacturing cost can be greatly reduced.

An internal conductor 610 a is inserted in an external conductor 610 b of the first coaxial waveguide formed within the cover body 300. Likewise, internal conductors 620 a to 640 a of a second to a fourth coaxial waveguide are inserted in external conductors 620 b to 640 b of the second to the fourth coaxial waveguide formed within the cover body 300. The tops of the external conductors 610 b to 640 b are covered by a lid 660. The internal conductor of each coaxial waveguide is made of copper having high heat conductivity.

A microwave is supplied from the microwave source 900 and transmitted to the first coaxial waveguide (internal and external conductors 610 a and 610 b) and the second coaxial waveguide (internal and external conductors 620 a and 620 b) through the fourth coaxial waveguide (internal and external conductors 640 a and 640 b) via the third coaxial waveguide (internal and external conductors 630 a and 630 b). The surfaces of the dielectric plates 305 are covered with metal films 305 a except a portion on which the microwave is incident from the first coaxial waveguide 610 and a portion from which the microwave is radiated. Accordingly, propagation of the microwave may not be disturbed by gaps between the dielectric plates 305 and adjacent members and, thus, the microwave can be stably introduced into the processing chamber.

The microwave radiated from the dielectric plates 305 becomes a surface wave and propagates on the surfaces of the metal electrodes 310, the metal covers 320 and the side cover 350 while its power is divided equally. Hereinafter, a surface wave propagating between a metal surface on an inner surface of the processing chamber and the plasma is referred to as a metal surface wave. In this configuration, the metal surface wave propagates on the entire ceiling surface, so that uniform plasma can be stably generated under the ceiling surface of the microwave plasma processing apparatus 10 in accordance with the present embodiment.

An octagonal groove 340 is formed on the side cover 350 so as to surround the eight dielectric plates 305. The octagonal groove 340 suppresses the metal surface wave propagating on the ceiling surface from propagating to the outside of the groove 340. Alternatively, a multiple number of grooves 340 may be formed in parallel or a protrusion may be formed instead of the groove 340. Here, the groove 340 or the protrusion is an example of a propagation stopping member.

With respect to one metal electrode 310, an area having centers of adjacent metal covers 320 as vertexes is referred to as a cell area (Cel) (see FIG. 2). On the ceiling surface, same patterns corresponding to eight cell areas (Cel) are regularly arranged.

A coolant supply source 910 is connected to a coolant pipe 910 a within the cover body. A coolant supplied from the coolant supply source 910 is circulated through the coolant pipe 910 a within the cover body and then returned back into the coolant supply source 910, so that the processing chamber 100 can be maintained at a desired temperature. A coolant pipe 910 b is inserted through the internal conductor 640 a of the fourth coaxial waveguide in a lengthwise direction thereof. By flowing the coolant through the coolant pipe 910 b, heating of the internal conductor 640 a is suppressed.

Desirably, a gap may not exist between the dielectric plate 305 and the cover body 300 or between the dielectric plate 305 and the metal electrode 310. If there exists a gap, a wavelength of the microwave propagating on the dielectric plate 305 may become unstable, which may have an adverse effect on plasma uniformity or load impedance when viewed from the coaxial waveguide. Furthermore, if there exists a large gap (equal to or larger than about 0.2 mm), an electric discharge may take place in the gap. For this reason, by tightening a nut 435, the dielectric plate 305 and the lower cover body 300 b is brought into contact with each other, and the dielectric plate 305 and the metal electrode 310 are also be brought into contact with each other.

If the nut 345 is tightened with an excessive torque, a stress may be applied to the dielectric plate 305, and the dielectric plate 305 may be broken. Even if the dielectric plate is not broken when the nut 435 is tightened, since a stress may be still applied to the dielectric plate 305 when a temperature of each member increases due to plasma generation, there is possibility that the dielectric plate 305 may be broken. Thus, in order to hold the metal electrode 310 by an appropriate force (slightly larger than a force for pressing the O-ring 220 and brining the dielectric plate 305 and the lower cover body 300 b into contact with each other) via the screw 380, a wave washer 430 b having an optimum elastic force is inserted between the nut 435 and the lower cover body 300 b. The nut 435 is not fully tightened until the wave washer 430 b becomes flat, and a deformation amount is maintained constant.

Although a washer 430 a is provided between the nut 435 and the wave washer 430 b, the washer 430 a may be omitted. Further, a washer 430 c is provided between the wave washer 430 b and the lower cover body 300 b. Typically, a gap is present between the first screw 380 and the cover body 300, and the gas within the main gas flow passage 330 flows into the first gas flow passage 310 a through the gap. If the amount of this gas is too much, the gas may be non-uniformly discharged through the first gas discharge holes 345 a. To prevent this problem, the gap between the washer 430 c and the first screw 380 is minimized and a thickness of the washer 430 c is increased, thus reducing a flow rate of a gas flowing through the outside of the first screw 380.

Now, optimizing configurations of the metal electrode 310 and its adjacent members for controlling the propagation of the metal surface wave in the above-described microwave plasma processing apparatus 10 will be described in further detail. First, an aspect ratio of the cell area (Cel) may be optimized. Second, the filling dielectric member 315 may be filled in the groove between the metal electrodes 310 and the metal cover 320 or the like. Third, the kinds, shapes and positions of the screws for fixing the metal electrodes 310 may be optimized.

(Aspect Ratio of Cell Area)

First, optimizing an aspect ratio of a cell area (Cel) will be explained. As shown in FIG. 2, the cell area (Cel) is a virtual area that divides the ceiling surface of the processing chamber 100. The cell area (Cel) is formed by two straight lines parallel to a diagonal line D1 of the metal electrode 310 and two straight lines parallel to a diagonal line D2 of the metal electrode 310. The cell area (Cel) is a minimum rectangular area including the metal electrode 310 and the dielectric plate 305.

In order to generate uniform plasma, distributions of standing waves of metal surface waves on the surfaces of the metal electrode and the metal cover are required to be uniform and there is a need of no great difference in the distributions of the standing waves in respective cases. For the purpose, the shapes of the metal electrode 310 and the dielectric plate 305 may be designed such that a ratio of a length of a long side of the cell area (Cel) to a length of a short side thereof may be about 1.2 or less.

If the cell area has a regular quadrilateral shape, same patterns of standing waves are formed on the surfaces of the metal electrode and the metal cover. It is because metal surface waves having same phases and same magnitudes are respectively supplied from corresponding positions around the metal electrode and the metal cover. FIGS. 3A and 3B provide simulation results showing patterns of the standing waves on the surfaces of the metal electrode 310 and the metal cover 320. White areas in the figures represent strong electric fields, while black areas indicate weak electric fields. In the figures, only a ¼ upper right area of the metal electrode 310, a ¼ lower left area of the metal cover 320 and a part of the dielectric plate 305 located therebetween are illustrated.

On the surface of the metal electrode, antinodes of the standing wave exist at positions A1, B1 and C1 of FIG. 3A. At positions A2, B2 and C2 of the metal cover surface corresponding to the positions A1, B1 and C1, antinodes of the standing wave having the same magnitudes as those of the standing wave on the surface of the metal electrode are found.

Meanwhile, if the cell area has a rectangular shape, different patterns of standing waves are formed on the surfaces of the metal electrode and the metal cover. It is because phases and magnitudes of the metal surface waves supplied from corresponding positions around the metal electrode 310 and the metal cover 320 are not same.

In comparison of the positions A1, B1 and C1 where the antinodes of the standing wave on the surface of the metal electrode exist and the positions A2, 32 and C2 where the antinodes of the standing wave on the surface of the metal cover exist, an electric field intensity at the position A2 is lower than an electric field intensity at the position A1; an electric field intensity at the position B2 is higher than an electric field intensity at the position B1; and an electric field intensity at the position C2 is lower than an electric field intensity at the position C1.

As can be seen from the above, if the symmetry of the standing waves of the metal surface waves on the surfaces of the metal electrode and the metal cover is poor, uniform control may become difficult, making it difficult to generate uniform plasma. In order to generate uniform plasma, an average value of an electric field intensity ratio of the metal surface waves at corresponding positions on the surfaces of the metal electrode and the metal cover is required to be equal to or smaller than about 1.5, more desirably, equal to or smaller than about 1.1.

FIG. 4 shows a relationship between an aspect ratio of a cell area and an electric field intensity ratio of metal surface waves at corresponding positions. FIG. 4 provides a result calculated by a simulation of electromagnetic field. Among the positions A1 (A2), B1 (B2) and C1 (C2) on the metal electrode 310 and the metal cover 320, a greater value of electric field intensity is divided by a smaller value, and the divided values are averaged for A, B and C and then expressed on a vertical axis. In case that a cell area is a regular quadrilateral (an aspect ratio of about 1), a length of one side of the cell is set to about 214 mm. In case that the aspect ratio is not about 1, the aspect ratio is varied while the area of the cell is maintained constant.

In case that the cell is a regular quadrilateral (an aspect ratio of about 1), the electric field intensity at all corresponding positions are same. Accordingly, an electric field intensity ratio is about 1. If the aspect ratio of the cell area increases, the electric field intensity ratio also increases.

As can be seen from the result of FIG. 4, in order to set the electric field intensity ratio to about 1.5 or less, the aspect ratio of the cell area needs to be equal to or less than about 1.2. In this way, the distributions of the standing waves of the metal surface waves on the surfaces of the metal electrode and the metal cover can be uniformized or approximated. As a result, there is no great difference in the distributions of the standing waves in each case, so that uniform plasma can be generated.

Further, in order to set the electric field intensity ratio to be equal to or less than about 1.1, the aspect ratio of the cell area needs to be equal to or less than about 1.1. In this way, the distributions of the standing waves of the metal surface waves on the surfaces of the metal electrode and the metal cover can be more uniformized or approximated. As a result, there is no great difference in the distributions of the standing waves in each case, so that uniform plasma can be generated.

(Burying Filling-Dielectric-Member)

Now, burying the filling dielectric member 315 in a gap between the metal electrode 310 and the metal cover 320 will be explained.

In a groove (Gap) between the metal electrode 310 and the metal cover 320 shown in FIG. 5( a), a gas is difficult to flow but readily stay therein. By way of example, in a plasma cleaning process, it may be difficult for a cleaning gas to enter the gap, and, thus, a film adhering to an inner surface of the gap may not be removed.

Further, since the groove-shaped gap is surrounded by walls in three directions, a plasma electron density may easily decrease, and, thus, it may become difficult to maintain stable plasma having constant density. Further, since metal surface waves are supplied from the dielectric plates 305 between the metal electrode 310 and the metal cover 320 or between the metal electrode 310 and the side cover 350, the entire plasma may become unstable and non-uniform unless the stable plasma having constant density is maintained in the gap.

Moreover, a microwave that has passed through the dielectric plate 305 may be split and the split microwaves may propagate on the surfaces of the metal electrode and the metal cover respectively. In the groove (Gap) shown in FIG. 5( b), intensity of electric fields E applied to a sheath region S may be non-uniformly distributed, so that there may be caused a difference in energy distribution of a metal surface wave M1 propagating on the surface of the metal electrode 310 and a metal surface wave M2 propagating on the surface of the metal cover 320 or the side cover 350.

In contrast, in the microwave plasma processing apparatus 10 in accordance with the present embodiment as shown in FIG. 1, the filling dielectric member 315 is filled in the groove (Gap). Accordingly, the groove between the metal electrode 310 and the metal cover 320 or between the metal electrode 310 and the side cover 350 may be filled and become flat. Thus, since there is no space into which the cleaning gas is difficult to flow, cleaning efficiency may be improved and maintenance may become easy. Furthermore, since there exists no space where unstable plasma with variable density is generated, an abnormal electric discharge may be prevented, so that uniform plasma can be stably generated. Further, as illustrated in FIG. 6A, since the microwave is supplied into the processing chamber via the filling dielectric member 315, intensity of electric fields applied to a sheath region S on the side of the metal electrode and a sheath region on the side of the metal cover (side cover) may not be non-uniformly distributed, so that energy can be equally divided to the metal surface wave MS1 propagating on the surface of the metal electrode 310 and the metal surface wave MS2 propagating on the surface of the metal cover 320 (or side cover 350). As a result, uniform plasma can be stably generated.

The filling dielectric member 315 may be provided so as to planarize the groove between the metal electrode 310 and the side cover 350 (or metal cover 320) in FIG. 6A. Alternatively, the filling dielectric member 315 may be protruded from the groove, as depicted in FIG. 6B. Otherwise, as illustrated in FIG. 6C, the filling dielectric member 315 may be buried in the groove. In any of these cases, a stay of a gas around the metal electrode can be prevented, and the filling dielectric member 315 may contribute to maintaining uniform and stable plasma. Moreover, in FIG. 6C, a protrusion 300 c having the same shape as or a similar shape to the metal electrode 310 is formed at a portion of the ceiling surface of the processing chamber (cover body 300) where the dielectric plate 305 is not provided. The filling dielectric member 315 is provided between the protrusion 300 c and the metal electrode 310. Here, the dielectric plate 305 and the filling dielectric member 315 may be formed as one body.

FIG. 7 illustrates one metal electrode 310 and its adjacent members. Edges of the metal electrode 310 is chamfered and inclined. The filling dielectric member 315 is a frame-shaped member provided on a bottom surface (plasma-facing side) of the dielectric plate 305 exposed at the outside of the metal electrode 310 so as to surround the outside of the metal electrode 310. The filling dielectric member 315 has the same octagonal shape as the dielectric plate 305. A protrusion 315 a is formed at the filling dielectric member 315 near a center of each side of the metal electrode 310. The filling dielectric member 315 is also slightly protruded near each vertex of the metal electrode 310 (protrusion 315 b). The protrusion 315 a near the center of each side of the metal electrode 310 is protruded toward the plasma more than the protrusion 315 b near the vertex of the metal electrode 310. In addition, the protrusions of the filling dielectric member 315 are arranged to have point symmetry with respect to the center of the metal electrode 310. With this configuration, a difference in distributions of electric field intensity around the metal electrode can be prevented and uniform plasma can be generated.

(Screws for Fixing the Metal Electrode)

Now, optimizing the kinds, shapes and positions of the screws for fixing the metal electrode 310 will be discussed. In FIG. 8, the metal electrode 310 included in a single cell area (Cel) is illustrated. FIG. 9 is a cross sectional view taken along a line 3-3 of FIG. 8.

A dielectric plate 305 and the metal electrode 310 shown in FIG. 9 are provided at positions where the vacuum and the atmosphere are isolated. Accordingly, inside an O-ring 220, a great pressure is applied to the dielectric plate 305 and the metal electrode 310 from the atmosphere side toward the vacuum side. Further, a great downward force is applied to the dielectric plate 305 and the metal electrode 310 by an elastic force of the O-ring 220. Accordingly, a gap may be easily formed between the dielectric plate 305 and the cover body 300.

If the size of the cell is increased, the number of cells per an apparatus may be reduced, and, thus, cost can be cut. If the size of the cell is increased, however, a gap may be more easily formed around the dielectric plate 305. Further, since heat from the plasma may also increase, the metal electrode 310 may be overheated.

A microwave cannot pass through a conductor. Thus, after propagating in the first coaxial waveguide 610, the microwave passes through the dielectric plate 305 and the filling dielectric member 315 and then is introduced into the processing space. On a design step, a design value of a microwave transmission path is previously predetermined in consideration of microwave transmission efficiency when a gap is not present. Actually, however, a small gap may be formed. In such a case, since a wavelength and a propagation speed of the microwave are varied due to the gap during a process, there is a difference between actual impedance and the design value. As a result, reflection of the microwave from the plasma would be increased, resulting in deterioration of energy supply efficiency of the microwave. Moreover, if the reflection of the microwave from the plasma increases, a standing wave ratio (peak ratio of standing wave) of the microwave may also increase within the first coaxial waveguide 610 shown in FIG. 9 and an electric discharge may be generated within the coaxial waveguide. As a result, the inside of the coaxial waveguide 610 may be heated or an abnormal electric discharge may be generated.

To solve the problem, in the metal electrode 310 in accordance with the present embodiment, the metal electrode 310 and the dielectric plate 305 are fixed to the cover body 300 while they are in contact with each other. That is, as depicted in FIGS. 8 and 9, the metal electrode 310 are firmly fixed to a ceiling surface within the cover body 300 by four first screws 380 and four second screws 390 different from the first screws 380. The four first screws 380 and the four second screws 390 fix the metal electrode 310 at axially symmetrical positions with respect to the center of the metal electrode 310. The four first screws 380 are located on diagonal lines D1 and D2 of the metal electrode 310.

The four second screws 390 fix the metal electrode 310 at positions equi-spaced from the center of the metal electrode 310 and different from the positions of the four first screws 380. The four second screws 390 are located at positions closer to the center of the metal electrode 310 than the four first screws 380. A diameter of each of the four first screws 380 is smaller than a diameter of each of the four second screws 390. The four first screws 380 and the four second screws 390 have symmetry with respect to the center of the metal electrode.

As stated above, by additionally installing the second screws 390 in addition to the first screws 380 and optimizing the positions and the diameters of the first and second screws 380 and 390, distribution of metal surface waves can be appropriately adjusted and more uniform plasma can be generated. Moreover, impedance can be appropriately adjusted and reflection of the microwave from the plasma can be reduced.

In case that the metal electrode 310 is of a regular quadrilateral shape, each of the four second screws 390 may be installed at a position equi-spaced from adjacent first screws 380 among the four equi-spaced first screws 380. To elaborate, as depicted in FIG. 8, the four second screws 390 may be provided on straight lines B1 and B2 connecting the center of the metal electrode 310 and vertexes of the cell area (Cel). The four first screws 380 and the four second screws 390 have point symmetry with respect to the center of the metal electrode.

The four second screws 390 may be exposed from the metal electrode 310 on the same plane as a plasma surface of the metal electrode 310, like the four first screws 380 shown in FIG. 1. Alternatively, the four second screws 390 may be configured to hold the dielectric plate 305 and the metal electrode 310 on the ceiling surface without being exposed to the plasma surface of the metal electrode 310. Furthermore, the number of the first screws 380 and the number of the second screws 390 may not be limited to four.

With this configuration, no gap or groove may be formed on the propagation path of the microwave. Accordingly, reflection of the microwave from the plasma can be reduced, and energy supply efficiency of the microwave can be improved. As a consequence, plasma having high electron density and high uniformity can be stably generated. Further, heat from the plasma can be transferred to the cover body 300 through the first and second screws 380 and 390. Moreover, in the present embodiment, since the diameters and the arrangement positions of the first and second screws are optimized, uniformity of an electric field of the metal electrode 310 can be achieved.

The various embodiments have been described with reference to the accompanying drawings, but the present invention is not limited thereto. It would be understood by those skilled in the art that various changes and modifications may be made without departing from the scope of the invention as defined in the following claims and it shall be understood that all modifications and embodiments conceived from the meaning and scope of the claims and their equivalents are included in the scope of the invention.

By way of example, the metal electrode 310 may have a polygonal shape other than a rhombus shape.

In the above embodiments, the microwave source 900 is described to output a microwave of about 915 MHz, a microwave source capable of outputting a microwave of, but not limited to, about 896 MHz, about 922 MHz or about 2.45 MHz may be used. Moreover, the microwave source is nothing more than an example of an electromagnetic wave source that generates an electromagnetic wave for exciting plasma. A magnetron or a high frequency power supply may also be used as long as they can output an electromagnetic wave equal to or higher than about 100 MHz.

The microwave plasma processing apparatus may perform various processes such as a film forming process, a diffusion process, an etching process, an ashing process and a plasma doping process for micro-processing a target object by plasma.

For example, the plasma processing apparatus in accordance with the present invention is capable of processing a large-size glass substrate, a circular silicon wafer or a square-shaped SOI (Silicon On Insulator) substrate. 

1. A plasma processing apparatus comprising: a processing chamber configured to excite therein a gas and perform a plasma process on a processing target object; an electromagnetic wave source installed outside the processing chamber and configured to output an electromagnetic wave; a dielectric plate provided adjacent to a ceiling surface within the processing chamber and configured to radiate the electromagnetic wave output from the electromagnetic wave source to the inside of the processing chamber; and a rhombus-shaped metal electrode provided on the side of a plasma-facing surface of the dielectric plate so as to be adjacent to the dielectric plate, wherein a part of the dielectric plate is exposed to the inside of the processing chamber at an outside of the metal electrode, and a cell area is defined as a virtual area that divides the ceiling surface of the processing chamber and is formed by two straight lines parallel to one diagonal line of the metal electrode and two straight lines parallel to the other diagonal line of the metal electrode and the cell area is a minimum rectangular area including the metal electrode and the dielectric plate, and shapes of the metal electrode and the dielectric plate are designed such that a ratio of a length of a long side of the cell area to a length of a short side thereof is equal or less than about 1.2.
 2. The plasma processing apparatus of claim 1, wherein the shapes of the metal electrode and the dielectric plate are designed such that the ratio of the length of the long side of the cell area to the length of the short side thereof is equal or less than about 1.1.
 3. A plasma processing apparatus comprising: a processing chamber configured to excite therein a gas and perform a plasma process on a processing target object; an electromagnetic wave source installed outside the processing chamber and configured to output an electromagnetic wave; a dielectric plate provided adjacent to a ceiling surface within the processing chamber and configured to radiate the electromagnetic wave output from the electromagnetic wave source to the inside of the processing chamber; a metal electrode provided on the side of a plasma-facing surface of the dielectric plate so as to be adjacent to the dielectric plate; and a metal cover having the same shape as or a similar shape to the metal electrode and provided on an area of the ceiling surface of the processing chamber where the dielectric plate is not provided, wherein a part of the dielectric plate is exposed to the inside of the processing chamber at an outside of the metal electrode, and a filling dielectric member is provided in a groove between the metal electrode and the metal cover.
 4. A plasma processing apparatus comprising: a processing chamber configured to excite therein a gas and perform a plasma process on a processing target object; an electromagnetic wave source provided outside the processing chamber and configured to output an electromagnetic wave; a dielectric plate provided adjacent to a ceiling surface within the processing chamber and configured to radiate the electromagnetic wave output from the electromagnetic wave source to the inside of the processing chamber; a metal electrode provided on the side of a plasma-facing surface of the dielectric plate so as to be adjacent to the dielectric plate; and a protrusion having the same shape as or a similar shape to the metal electrode and provided on an area of the ceiling surface of the processing chamber where the dielectric plate is not provided, wherein a part of the dielectric plate is exposed to the inside of the processing chamber at an outside of the metal electrode, and a filling dielectric member is provided in a groove between the metal electrode and the protrusion.
 5. The plasma processing apparatus of claim 3, wherein a side cover is provided around the metal electrode, and the filling dielectric member is provided in a groove between the metal electrode and the side cover.
 6. The plasma processing apparatus of claim 3, wherein the filling dielectric member is buried in the groove, or is used to planarize the groove, or is protruded from the groove.
 7. The plasma processing apparatus of claim 6, wherein the filling dielectric member is provided so as to surround the outside of the metal electrode and has protruded portions from the groove.
 8. The plasma processing apparatus of claim 7, wherein the filling dielectric member is protruded from the groove in the vicinity of a center of each side of the metal electrode.
 9. The plasma processing apparatus of claim 7, wherein the protruded portion of the filling dielectric member in the vicinity of a center of each side of the metal electrode is higher than the protruded portion in the vicinity of a vertex of the metal electrode.
 10. The plasma processing apparatus of claim 7, wherein the protruded portions of the filling dielectric member are arranged to have point symmetry with respect to a center of the metal electrode.
 11. The plasma processing apparatus of claim 3, wherein the filling dielectric member is made of the same material as that of the dielectric plate.
 12. A plasma processing apparatus comprising: a processing chamber configured to excite therein a gas and perform a plasma process on a processing target object; an electromagnetic wave source installed outside the processing chamber and configured to output an electromagnetic wave; a dielectric plate provided adjacent to a ceiling surface within the processing chamber and configured to radiate the electromagnetic wave output from the electromagnetic wave source to the inside of the processing chamber; and a metal electrode provided on the side of a plasma-facing surface of the dielectric plate so as to be adjacent to the dielectric plate, wherein a part of the dielectric plate is exposed to the inside of the processing chamber at an outside of the metal electrode, the metal electrode is fixed to the ceiling surface within the processing chamber by a plurality of first screws and a plurality of second screws different from the first screws, the plurality of first screws are configured to fix the metal electrode at positions having point symmetry with respect to a center of the metal electrode, and the plurality of second screws are arranged at positions having point symmetry with respect to the center of the metal electrode and are configured to fix the metal electrode at positions different from the positions of the plurality of first screws.
 13. The plasma processing apparatus of claim 12, wherein diameters of the plurality of first screws are smaller than diameters of the plurality of second screws.
 14. The plasma processing apparatus of claim 12, wherein the number of the plurality of first screws is four and the four first screws are located on diagonal lines of the metal electrode.
 15. The plasma processing apparatus of claim 14, wherein the number of the plurality of second screws is four and the four second screws are located at positions closer to the center of the metal electrode than the four first screws.
 16. The plasma processing apparatus of claim 15, wherein if the metal electrode is of a regular quadrilateral shape, each of the four second screws is installed at a position equi-spaced from two adjacent first screws among the four first screws.
 17. The plasma processing apparatus of claim 1, wherein the dielectric plate is exposed in a substantially strip shape to the inside of the processing chamber at the outside of the metal electrode.
 18. The plasma processing apparatus of claim 1, wherein the number of the dielectric plate and the number of the metal electrode are plural, the dielectric plates are sandwiched between the metal electrodes and the ceiling surface of the processing chamber, and the metal electrodes are regularly arranged on the ceiling surface such that vertexes of adjacent metal electrodes are positioned closest to each other. 